This invention relates to the art of simultaneous telecommunication between radio stations, and more particularly to a radio communication system for simultaneous telecommunication between radio stations, using a carrier of single frequency for carrying audio information signals, in which a time division pulse signal of low frequency is generated for the transmission and reception of the audio information signals in time division fashion, and the audio information signal received in time division fashion is reproduced in a unique manner to improve the intelligibility of the reproduced audio information.
In the exchange of information between a calling station and a called station by radio communication units using a carrier of single frequency, the operator in the called station manipulates the radio communication unit upon termination of transmission of a message from the calling station so that he can now transmit his message to the calling station. In such manner of information exchange, however, free conversations like those over a telephone line cannot be attained. In other words, the operator talking by the radio communication unit in the calling or called station cannot say yes or no while the other is talking. Further, at least one hand of the operator in each of the two stations is occupied for the exclusive purpose of switch-over of transmission and reception of audio information signals since such switch-over must be manually carried out. Thus, there has been a strong demand for a radio communication system for simultaneous telecommunication which is capable of automatic switch-over of transmission and reception of audio information signals. However, in the field of information exchange between radio stations, a radio communication system for simultaneous telecommunication capable of automatic switch-over of audio information signals transmitted from and received by radio communication units using a carrier of single frequency has not yet been realized for the reasons described in detail below.
A radio communication system for simultaneous telecommunication capable of automatic switch-over of audio information signals transmitted from and received by radio communication units using a carrier of single frequency can be realized and successfully put into practical use when the transmission and reception of the audio information signals in the calling station and the called station are carried out in synchronous fashion utilizing a time division pulse signal of high frequency. Such a system cannot however be realized due to the fact that a high frequency signal used in such manner for the purpose of time division transmission and reception does not inevitably fall within the limited frequency band prescribed in the laws including the Radio Law.
Resorting to time division utilizing a low frequency is thus the only method which can achieve the desired time division with a frequency which falls within the limited frequency band prescribed in the said laws. However, the time division with a low frequency results in extreme deterioration of the intelligibility of reproduced audio information due to the discontinuity of the audio information signal which results from the time division by the low frequency, and thus, difficulty is encountered in the realization of the desired simultaneous telecommunication. For the reasons above pointed out, it has heretofore been unable to realize simultaneous telecommunication using a carrier of single frequency and utilizing a time division signal of low frequency for the automatic switch-over of transmitted and received audio information signals.